This invention relates to a thermometer for detecting the temperature by utilizing the dependency of resistance on the temperature of a resistance bulb, and more particularly, to a digital thermometer for detecting the changes in the resistance by the variation of oscillation frequency of an oscillator.
It is generally known to measure the temperature from an oscillation frequency by using a resistance bulb in part of a CR oscillation circuit.
The oscillation frequency F is, in a practically negligible error range, given as follows when using a proportional constant K: ##EQU1##
If the resistance bulb is a thermistor, the relation between the resistance R of thermistor and absolute temperature T is theoretically expressed as follows: ##EQU2## where Ro is the thermistor resistance when the absolute temperature is To, and B is the thermistor constant. From equations (1) and (2), the absolute temperature T is expressed in terms of the oscillation frequency F as shown in equation (3): ##EQU3## where Fo is the oscillation frequency at absolute temperature To.
Therefore, by measuring the oscillation frequency, the temperature is known.
In the conventionally known thermometers, however, many circuit elements were needed, and it was unsuited for reducing the cost.